This invention relates to gain and offset control apparatus for reading data from an optical disk, preferably a magneto-optical disk, and more particularly, to such apparatus that is controlled in response to reference patterns read from the disk.
Optical disks are recorded with a spiral track or with concentric tracks in which information is represented by pits. Such disks are driven either at a constant angular velocity (CAV), whereby the density of the pits generally decreases at those tracks located at a greater radius from the center, or at a constant linear velocity (CLV), whereby the rotary speed of the disk increases as the radius of the track being scanned increases such that the density of the pits generally is constant over the entire disk.
The information recorded on an optical disk generally includes useful information, such as audio information for an audio disk, video and video/audio information for a video disk and computer file data for a computer disk (such as a CD ROM). In addition to such useful information, optical disks include address information to permit a user to access a desired portion of the disk, such as a desired data sector, and servo information which is used for tracking control (i.e. to assure "center-line" tracking of a recording or pickup head) and to synchronize the clock of the disk drive with the actual speed of the disk. Servo information may be recorded in a pre-existing groove that is formed continuously along the track of a spiral-track optical disk or, alternatively, the servo information may be distributed along the spiral or concentric tracks in discrete servo areas. For example, when manufacturing the optical disk, such discrete servo areas may be pre-formed as part of the manufacturing process.
When reproducing information from an optical disk, whether that disk is a playback-only (or read-only) disk, a write-once disk, a recordable disk (such as a magneto-optical disk) or a hybrid disk having read-only and recordable areas, the amplitude as well as the offset of the reproduced signals (as used herein, "offset" refers to the drift in the center or "zero" level of the reproduced signals) may fluctuate because of fluctuations inherent in the optical system. Such optical system fluctuations are attributed to fluctuations in the disk speed, fluctuations occasioned by variations or changes in circuit parameters or fluctuations due to changes in the reflectance or other optical properties of the recording medium or changes in the Kerr rotation angle. While fluctuations in circuit parameters, for example, from one disk drive to another, can be compensated by performing suitable gain adjustments as part of the disk drive manufacturing operation, such "factory adjustments" are time consuming, require the skill of expert technicians and often are not completely successful. Generally, such factory adjustments are not helpful in eliminating drive-to-drive fluctuations in the reflectance or optical properties of the recording medium or in changes in the Kerr rotation angle. Additionally, variations in the amplitude of the reproduced signals that may be caused simply by age of the disk drive, or amplitude variations due to small defects in the recording medium or due to the presence of dust or dirt on the recording medium certainly cannot be compensated or even predicted by factory adjustments.
It is important, therefore, to provide a dynamic technique for detecting the amplitude of data recorded on an optical disk, such as a magneto-optical disk, even though the signal levels reproduced from such data may change, or may be subject to fluctuations from the nominal factory pre-set values thereof.